You can assume it's 5160...but it may not be. Let's just hope it is, and talk about 5160 heat treatment. You are using a forge, correct? That's not ideal for heat treatment, but with 5160, you can get away with it, because 5160 is relatively simple to HT. This is what I would do (quenching in 400°F oil is not needed at all, that is a particular sort of HT that is not necessary, but to do it would require an oil with a flash point above 400°F). For 5160, quench oil can be AAA or similar medium speed oil, and canola warmed to 130°F works very well, so if you're not inclined to purchase the commercial quenchant (I would if I were going to be doing this much in the future), use the canola warmed up. Do you already have the knife cut out of the leaf? If not I would anneal the leaf spring, to make it soft, then cut the knife. Annealing (a simple anneal) 5160 would be to bring it up to ~1500F and then stick it in a bucket of wood ash/vermiculite/sand overnight. Or you can bring it up to ~1500F in your forge, and then turn the forge off and let it cool in the forge. I would NOT do that anneal with higher carbon steels, but for 5160 and 1084 it will work. If you've already cut the knife....forget the anneal. You need to normalize the steel first. It was a leaf spring in a previous life...we now want the phoenix to be reborn into a new creature. We need to "reset" the steel, so to speak, when coming from a leaf spring especially. That way we know exactly what we're working with going into the hardening/quench. Normalizing by definition is done only once, and is at 1600°F for this particular alloy. I would say soak at this temp for a few minutes, if you can with your forge. Air cool only. Normalizing is now complete, but we can refine the "grain" by "thermal cycles". The temps chosen are up to you, but you can do 1550F, then 1500F, then 1450F, all with air cooling only. You can change those temps around a bit, even doing 3 cycles at around 1500F. Up to you. I like to drop the temps myself each cycle. To harden, warm your canola oil to 130°F. Insert your blade (best to have some sort of scale/decarb protection like ATP641 coating) into your forge when it has settled at 1525°F (I would wait a little while and let the forge settle into that temperature, rather than inserting the blade as soon as it reaches that temp). If you can, soak the blade for 5 minutes, just to ensure a nice solution, and then quench in your oil. Be sure to agitate the blade up and down, but never side to side (warping). Once the blade has cooled to the oil temp, you can remove. Actually, if you detect a warp immediately after inserting into the oil, then you can straighten it as it falls in temperature down to ~400°F with gloved hands, then back into the oil until it cools to oil temp. Once cooled to room temp, you can check the hardness with a file skate test (which is not an end all be all test...but can tell us if we at least hardened it). Watch out for the decarb layer that is/might be on the steel surface. This will fool you into thinking your attempt was a failure. It will feel like soft steel, and it soft, but only on the very surface. File a little more, and you should reach hard steel underneath. Scale is the hard black stuff (Carbon) that came out of the steels surface (causing the carbon depleted layer...decarb). For tempering, there are differing opinions on time, but I like to do 2 hours each cycle, for at least 2 cycles, no more than 3. Some will temper only 1 hour, but I'll do 2. Maybe overkill, but certainly will not hurt anything. I'm not too familiar with 5160 temper numbers, but going by some charts (5160 temper charts are all over the place, take a look online), 300°f is ~63HRC, 350°F is ~61HRC, 400°F is ~60HRC, 450°F~58HRC. I would stay at or above 60HRC with 5160. No need to edge quench this steel. It's very tough already, and a fully hardened blade in this steel is very very tough. No need to edge quench and leave a soft spine. If you wanted a softer spine, best way is to fully harden then draw back the spine using a torch. But really not needed at all. No need to triple quench this steel, either. You can, but not needed if all was done right the first go around. Often when people triple quench, they are only putting in solution a small amount of carbon each quench. If you can control the forge temp accurately, and allow the steel to soak for maybe 5 minutes or so, you can make an excellent blade that is as it should be. Hopefully that will help get you started. 5160 is a fairly simple steel, and does not have excess carbon that needs special attention. It is relatively forgiving in HT, and even less than optimal equipment can produce a good knife.
